High-pressure discharge lamp lighting device

ABSTRACT

The high-pressure discharge lamp lighting device of the invention includes a lighting circuit for supplying an alternating current to a high-pressure discharge lamp to cause lighting, the high-pressure discharge lamp having an arc tube in which a halogen material is enclosed and a pair of electrodes is disposed, and each electrode having a protuberance at a tip thereof. A frequency of the alternating current varies without dependency on operating data that varies as a lighting time of the high-pressure discharge lamp elapses.

TECHNICAL FIELD

The present invention relates to a high-pressure discharge lamp lightingdevice, a high-pressure discharge lamp apparatus, a projector-type imagedisplay device, and a high-pressure discharge lamp lighting method, andrelates in particular to a high-pressure discharge lamp lighting devicethat causes current to flow in a high-pressure discharge lamp causinglighting.

BACKGROUND ART

Among high-pressure discharge lamps, the high-pressure mercury lamp,which is used as a light source in projector-type image display devicessuch as liquid crystal projectors and the like, has in recent years beenattracting particular attention (see Japanese Laid-open PatentApplication No. H4-303592).

Generally, the high-pressure mercury lamp has a pair of opposingelectrodes disposed inside an arc tube enclosing a halogen material, aninert gas, and mercury. A high-pressure discharge lamp lighting deviceapplies a predetermined high voltage pulse to the high-pressuredischarge lamp to give dielectric breakdown between the electrodes, andsubsequently causes an alternating current of a predetermined frequencyto flow thereby causing the lamp to light.

The life of such high-pressure mercury lamps is of the order of 2000hours to 3000 hours.

Liquid crystal projectors in the past were used mainly in schoolclassrooms, conference rooms, and the like, but in recent years havebecome increasingly popular with ordinary households.

Liquid crystal projectors use as their light source what is, withdischarge being caused in the short gap between the electrodes togenerate light, essentially a point light source. Consequently, imagestability is strongly influenced by discharge stability. For thisreason, lamp constructions and lighting methods for ensuring dischargestability in discharge lamps, such as the constructions and methodsdescribed below, have been in use for some time.

The high-pressure mercury lamps currently in use in liquid crystalprojectors generally include, besides mercury, a halogen material as oneof the enclosed materials. This is because the halogen cycle of theenclosed halogen material prevents the occurrence of lamp blackening dueto the electrode ingredient tungsten evaporating during lighting. Thishalogen cycle operation is very effective in the prevention of lampblackening. However, as the cycle continues, tungsten transferred to thetips of the electrodes via the cycle accumulates thereon, formingprotuberances. The growth of such protuberances can be controlled byvarying the frequency of the supplied alternating current, a methoddisclosed in Japanese Laid-open Patent Application No. 2001-312997 andin Japanese Laid-open Patent Application No. 2003-338394. Using thismethod, stability in the luminance when the lamp is dispatched isachieved by shortening the distance between the electrodes through theformation of suitable protuberances to make a point light source.

Further, growth of the protuberances during the first 100 to 500 hoursor thereabouts of the life can be suppressed by varying the frequency ofthe supplied AC current, a method which is disclosed in JapaneseLaid-open Patent Application No. 2003-338394. Using this method, theexcessive growth of the protuberances generated in the first 100 to 500hours or thereabouts of the life can be suppressed, and stability inintensity is achieved. The method involves detecting a discharge voltageand, with the voltage variation as a reference, performing feedbackcontrol to restore the tips of the electrodes.

DISCLOSURE OF THE INVENTION

However, in recent years a new problem has arisen as a result of therange of applications for liquid crystal projectors displays expandingto include home theater systems capable of wide screen display an ofdisplaying television pictures. The projectors used principally inschool classrooms and conference rooms generally displayed still images,and were in use for a maximum of a few hours each day. The projectorsused as TV displays or home theaters, on the other hand, generallydisplay moving pictures. Moreover, such projectors are usedcontinuously. Hence, it can be assumed that a period of use isincomparably longer than that of previous technologies. Moreover, therequirement for subtle variations in brightness, color reproduction, andthe like is stricter for images displayed on a home theater system andfor television pictures than for images displayed on a computer screen.Long-term stability is therefore required in the lamp. Consequently, the2000 to 3000 hour life of the projectors that were used mainly in theschool classroom and conference rooms is insufficient, and a lifeseveral times that of previous lamps is required. Furthermore, colorreproduction, brightness, and the like are required to be exceedinglystable for the duration of this longer life.

However, application of these previously available lighting methods indevices such as home theater systems capable of realizing televisionscreens and large screens did not enable a long life (6000 hours, forinstance) and stable color reproduction and brightness to be obtained.Analysis of the reasons for this revealed the following. Protuberancegrowth at the tips of the electrodes is marked in the initial period oflighting, but subsequently, in the period between 2000 hours and 3000hours of lighting, instead of further protuberance growth there is aslowly progressing size reduction, and the gap between the electrodeswidens. Because of this, the discharge region gradually expands and theintensity of the optical device drops. In the lamp lighting method ofprevious lamps this was controlled by detecting the discharge voltageand varying the frequency of the supplied alternating current, but thevariation of the discharge voltage in this process is slow, and such acontrol method does not work effectively. Further, the optimumconditions for controlling the protuberance growth and contraction varybecause electrical properties of the lamp alter slowly as its drivingperiod progresses. For reasons such as this, in the period spanningbetween 2000 and 3000 hours after lamp driving was begun, it wasdifficult to perfectly maintain the initial form of the electrodes bymeans of the control method for keeping the same driving frequencycondition.

Moreover, there is the issue of stability for the subtly varyingproperties such as color reproduction and brightness of the displayedimage. As previously indicated, in displays such as home theater systemscapable of realizing television pictures and large screens, long-termstability is required. Analysis of this problem reveals that the key tomaintaining stability is to maintain the form of the electrode tips. Thecentral point of emission from the light source is located in the lampelectrode gap. The brightness and color reproduction of the projectorare influenced by the size of the central point of emission, and thelocation of the same point is important because of its relationship withthe optical axis of the optical system. As a consequence, if the form ofthe electrodes is maintained, it is possible to maintain stability ofproperties such as the subtle variations in image color reproduction andbrightness.

It is an object of the present invention to provide a high-pressuredischarge lamp lighting device, a high-pressure discharge lampapparatus, a projection-type image display apparatus, and ahigh-pressure discharge lamp lighting method all of which are capable ofimproving a life that is determined as the period before a certain dropin intensity, and of maintaining over the life stability with respect toproperties such as the subtle variation of color reproduction,brightness and the like.

In order to achieve this object the high-pressure discharge lamplighting device of the invention includes: a lighting circuit operableto supply an alternating current to a high-pressure discharge lamp tocause lighting, the high-pressure discharge lamp having an arc tube inwhich a halogen material is enclosed and a pair of electrodes isdisposed, and each electrode having a protuberance at a tip thereof, anda frequency controlling unit operable to vary a frequency of thealternating current without dependency on operating data that varieswith an elapsed lighting time of the high-pressure discharge lamp.

Here “protuberance” is used to mean a part formed during thehigh-pressure discharge lamp manufacturing process, particularly duringan aging process in the manufacturing process, and during an initiallighting period of the finished product (the first 100 hundred hours oflighting or less). Further, “operating data that varies as a lightingtime elapses” is used to mean measurable operating data that alters withan elapsed lighting time of the high-pressure discharge lamp. Such dataincludes high-pressure discharge lamp voltage values, current values,luminance values of the optical device, the electrode gap distance (arclength), and the temperature of the arc tube, (particularly thetemperature of the upper part of the arc tube that is the operatingtemperature).

Here, the frequency controlling unit may vary the frequency during apredetermined period in a regular or irregular manner without dependencyon the operating data, and may repeat the predetermined periodconsecutively or intermittently.

Here, the frequency controlling unit may vary the frequency continuouslyduring the predetermined period.

Here, the frequency controlling unit may switch the frequencyintermittently among two or mom values during the predetermined period.

Here, the predetermined period may include one or more variable periodsduring which the frequency is varied continuously and one or more fixedperiods during which the frequency is fixed.

Here, the frequency controlling unit may alternate a variable period anda fixed period, varying the frequency continuously during the variableperiod without dependency on the operating data, and fixing thefrequency during the fixed period.

Here, the frequency controlling unit may switch the frequencyintermittently among two or more different values.

Here, the frequency controlling unit may constantly vary the frequencyin a regular or irregular manner without dependency on the operatingdata.

Here, the frequency may be varied between a predetermined maximumfrequency and a predetermined minimum frequency that is at least 70 Hz.

The present invention further includes a high-pressure discharge lampapparatus having: a high-pressure discharge lamp having an are tube inwhich a halogen material is enclosed and a pair of electrodes isdisposed, each electrode having a protuberance at a tip thereof; and thehigh-pressure lamp lighting device.

Moreover, the present invention further includes a projector-type imagedisplay apparatus including the high-pressure discharge lamp apparatus.

Moreover, the present invention further includes a high-pressuredischarge lamp lighting device having: a lighting circuit operable tosupply an alternating current to a high-pressure discharge lamp to causelighting, the high-pressure discharge lamp having an are tube in which ahalogen material is enclosed and a pair of electrodes is disposed, andeach electrode having a protuberance at a tip thereof and a frequencycontrolling unit operable to vary a frequency of the alternating currentin a regular or irregular manner during a predetermined period, andrepeat the predetermined period consecutively or intermittently.

Here, the frequency controlling unit may vary the frequency continuouslyduring the predetermined period.

Here, the frequency controlling unit may switch the frequencyintermittently among two or more values during the predetermined period.

Here, the predetermined period may include one or more variable periodsduring which the frequency is varied continuously and one or more fixedperiods during which the frequency is fixed.

The present invention further includes a high-pressure discharge lamplighting device having: a lighting circuit operable to supply analternating current to a high-pressure discharge lamp to cause lighting,the high-pressure discharge lamp having an arc tube in which a halogenmaterial is enclosed and a pair of electrodes is disposed, and eachelectrode having a protuberance at a tip thereof; and a frequencycontrolling unit operable to alternate a variable period and a fixedperiod, the frequency controlling unit varying a frequency of thealternating current continuously according to a predetermined rule inthe variable period, and fixing the frequency in the constant period.

Moreover, the present invention further includes a high-pressuredischarge lamp lighting device having: a lighting circuit a lightingcircuit operable to supply an alternating current to a high-pressuredischarge lamp to cause lighting, the high-pressure discharge lamphaving an arc tube in which a halogen material is enclosed and a pair ofelectrodes is disposed, and each electrode having a protuberance at atip thereof; and a frequency controlling unit operable to constantlyvary a frequency of the alternating current according to a predeterminedrule.

Moreover, the present invention further includes a high-pressuredischarge lamp lighting method having: a lighting step of supplying analternating current to a high-pressure discharge lamp to cause lighting,the high-pressure discharge lamp having an arc tube in which a halogenmaterial is enclosed and a pair of electrodes is disposed, and eachelectrode having a protuberance at a tip thereof; and a frequencycontrolling step of varying a frequency of the alternating currentwithout dependency on operating data that varies with an elapsedlighting time of the high-pressure discharge lamp.

Here, in the frequency controlling step, the frequency may be variedduring a predetermined period in a regular or irregular manner withoutdependency on the operating data, and the predetermined period may berepeated consecutively or intermittently.

Here, in the frequency controlling step, a variable period and a fixedperiod may be alternated, the frequency being varied continuously duringthe variable period without dependency on the operating data, and beingfixed during the fixed period.

Here, in the frequency controlling step, the frequency may be switchedintermittently among two or more different values.

Here, in the frequency controlling step, the frequency is varied in aregular or irregular manner without dependency on the operating data.

The present invention further includes a high-pressure discharge lamplighting method having: a lighting step of supplying an alternatingcurrent to a high-pressure discharge lamp to cause lighting, thehigh-pressure discharge lamp having an arc tube in which a halogenmaterial is enclosed and a pair of electrodes is disposed, and eachelectrode having a protuberance at a tip thereof; and a frequencycontrolling step of varying a frequency of the alternating current in aregular or irregular manner during a predetermined period, and repeatthe predetermined period consecutively or intermittently.

Moreover, the present invention includes a high-pressure discharge lamplighting method having: a lighting step of supplying an alternatingcurrent to a high-pressure discharge lamp to cause lighting, thehigh-pressure discharge lamp having an are tube in which a halogenmaterial is enclosed and a pair of electrodes is disposed, and eachelectrode having a protuberance at a tip thereof; and a frequencycontrolling step of alternating a variable period and a fixed period,the frequency controlling unit varying a frequency of the alternatingcurrent continuously according to a predetermined rule in the variableperiod, and fixing the frequency in the constant period.

Moreover, the present invention includes a high-pressure discharge lamplighting method having: a lighting step of supplying an alternatingcurrent to a high-pressure discharge lamp to cause lighting, thehigh-pressure discharge lamp having an arc tube in which a halogenmaterial is enclosed and a pair of electrodes is disposed, and eachelectrode having a protuberance at a tip thereof; and a frequencycontrolling step of constantly varying a frequency of the alternatingcurrent according to a predetermined rule.

According to the present invention, it is possible to provide ahigh-pressure discharge lamp lighting device, a high-pressure lampdevice, a projection type image display device and a high-pressuredischarge lamp lighting method all of which are capable of improving alife that is determined as the period before a certain drop inluminance, and of maintaining over the life the stability of propertiessuch as the subtle variation of color reproduction and brightness.

Further, variation in the luminance caused by variation in the form ofthe protuberances of the electrode tips during constant lighting issuppressed, enabling a previously unreachable life of the order of 3000hours or more to be achieved. Consequently, the life will be limited byother factors, such a drop in transparency due to clouding of the lampcasing tube or a drop in transparency due to deformation of the lampcasing tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general structure of a high-pressure mercury lamp 100;

FIG. 2 is perspective view with a cut-away section and shows thestructure of a lamp unit 200 in which the high-pressure mercury lamp 100is used;

FIG. 3 is a block diagram showing the structure of a lighting device300;

FIG. 4A is a graph showing relationships between accumulated lightingtime and a luminance sustain ratio from lighting tests for which aconventional method was used; FIG. 4B is a graph showing relationshipsbetween accumulated lighting time and a lamp voltage from lighting testsfor which a conventional method was used;

FIGS. 5A-5C are generalized diagrams showing variation of electrodeprotuberances over time in a lamp of the prior art using a fixedfrequency of 170 Hz; 5A shows the form of the electrode protuberancesbefore lighting begins; 5B shows the form of the electrode protuberancesin the initial stages of the life; and FIG. C shows the form of theelectrodes in the middle and latter stages of the life;

FIG. 6 includes a lower graph and an upper graph; the upper graph showsthe temporal variation of the wave form of a square-wave alternatingcurrent; the lower graph corresponds to the upper graph and shows thetemporal variation of the frequency of the square wave alternatingcurrent;

FIG. 7A is a graph showing relationships between accumulated lightingtime and a luminance sustain ratio from lighting tests for which aconventional method was used; FIG. 7B is a graph showing relationshipsbetween accumulated lighting time and lamp voltage from lighting testsfor which a conventional method was used;

FIG. 8 is a table showing luminance transition results from a lightingtest;

FIG. 9 is a table showing voltage transition results from a lightingtest;

FIG. 10 is a block diagram showing the structure of a liquid crystalprojector;

FIG. 11 shows the temporal variation of the wave form of the square-wavealternating current for a modified example;

FIG. 12 shows the temporal variation of the wave form of the square-wavealternating current for a modified example;

FIG. 13 shows the temporal variation of the wave form of the square-wavealternating can for a modified example;

FIG. 14 shows the temporal variation of the wave form of the square-wavealternating current for a modified example;

FIG. 15 shows the temporal variation of the wave form of the square-wavealternating current for a modified example;

FIG. 16 is a block diagram showing the structure of the lighting device301 for a modified example;

FIG. 17 shows the temporal variation of the frequency of the square-wavealternating current for a modified example;

FIG. 18 shows the temporal variation of the frequency of the square-wavealternating current for a modified example;

FIG. 19 shows the temporal variation of the frequency of the square-wavealternating current for a modified example; and

FIG. 20 shows the temporal variation of the frequency of the square-wavealternating current for a modified example.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

An embodiment of the invention is described in detail below withreference to the drawings.

1. Structure of a High-Pressure Mercury Lamp

As one example of a high-pressure discharge lamp, the structure of ahigh-pressure mercury lamp is described below.

FIG. 1 shows a general structure of a high-pressure mercury lamp 100with a 130 W power rating (hereinafter referred to as “the lamp”).

As shown in FIG. 1, an arc tube 101 has an external casing manufacturedfrom quartz glass, and includes an emission section 101 a and sealsections 101 b and 101 c that are provided at the ends of the emissionsection 101 a.

The emission section 101 a is of a substantially spheroidal form, andencloses, in an internal discharge space (a light emission space) 108thereof mercury 109 that is a light emitting material, an inert gas suchas argon (Ar), Krypton (Kr), Xenon (Xe), or the like, and for aidingstart-up, a halogen material such as Iodine (I), Bromide (Br), or thelike. Further, inside the emission section 101 a, a pair of tungsten (W)electrodes 102 and 103 are disposed substantially opposite one another.

The quantity of enclosed mercury 109 is set to be in a range of 150mg/cm³ to 650 mg/cm³ per unit volume inclusive in the arc tube 101, andthe pressure of the inert gas when the lamp is cool is set to be a rangeof 0.01 MPa to 1 MPa inclusive.

The halogen material has a function of returning tungsten caused toevaporate from the electrodes 102 and 103 due to the high temperaturewhen the lamp 100 is operating to the electrodes 102 and 103 in aprocess known as the halogen cycle. In order to make the cycle functioneffectively, it is desirable that the quantity of enclosed bromide is ina range of 1*10⁻¹⁰ mol/cm³ to 1*10⁻⁴ mol/cm³ inclusive, and preferablethat it is in a range of 1*10⁻⁹ mol/cm³ to 1*10⁻⁵ mol/cm³ inclusive.

The gap between the tips of the electrodes 102 and 103, which is theelectrode gap distance De, is set in a range of 0.5 mm to 2.0 mminclusive. Note that the electrodes 102 and 103 of the embodimentalready have protuberances 124 and 134 formed to a certain extent at thecompletion of manufacture, and the electrode gap distance De istherefore the gap between the tips of the protuberances 124 and 134.

The electrodes 102 and 103 (102 a and 103 b) are electrically connectedto external molybdenum leads 106 and 107 via pieces of molybdenum foil104 and 105, and the external molybdenum leads 106 and 107 respectivelyextend from an end surface of each of the seal sections 101 b and 101 cto the exterior of the arc tube 101.

2. Structure of Lamp Unit

FIG. 2 is perspective view with a cut-away section and shows thestructure of part of the lamp unit (a high-pressure discharge lampapparatus) 200.

The lamp unit 200 includes the lamp 100 and a high-pressure dischargelamp lighting device (not shown in FIG. 2) for causing the lamp 100 tolight, and a concave mirror 203 as a reflector (a reflective material)for reflecting light emitted from the lamp 100.

One end of the arc tube 101 (See FIG. 1) has a base 201 fitted to it,and the lamp 100 is fitted into the concave mirror 203 via a spacer 202.This fitting involves adjusting the components in such a way that thelength direction central axis of the are tube 101 and the optical axisof the concave mirror 203 are substantially aligned, and the position ofthe discharge arc of the lamp 100 substantially matches the focal pointof the concave mirror 203.

Power is supplied to the lead 107 (see FIG. 1) of the base 201 side ofthe lamp 100 via a terminal 204. Power is supplied to the other lead 106via a lead 205 that passes to the exterior through a hole 206 piercedthrough the concave mirror 203.

3. Structure of the Lighting Device

FIG. 3 is a block diagram showing the structure of a lighting device 300that causes the lamp 100 to light.

As shown in FIG. 3, the lighting device 300 (high-pressure dischargelamp lighting device) is composed of a DC power source 302, a DC/DCconverter 304, a DC/AC inverter 306, a high voltage generator 308, acontrol unit 310, a current detector 312, a voltage detector 314, and aprogrammable oscillator 316.

The DC power source 302 includes, for instance, a rectifying circuit,and generates direct current from domestic-use 100 V AC.

The DC/DC converter 304 supplies a direct current of a predeterminedmagnitude to the DC/AC inverter 306.

The DC/AC inverter 306 generates a square-wave alternating current of apredetermined frequency based on a control signal transmitted from thecontrol unit 310, and transmits the generated square-wave alternatingcurrent to the high voltage generation device 308.

The high voltage generator 308 includes, for instance, a transformer,generates a high voltage, and applies the generated high voltage to thelamp 100.

The control unit 310 collectively controls elements such as the DC/DCconverter 304, the DC/AC inverter 306.

The current detector 312 detects the current in the lamp 100, and thevoltage detector 314 detects the voltage in the lamp 100.

The programmable oscillator 316 generates a square-wave alternatingcurrent of a predetermined frequency based on a predetermined program.With reference to this wave shape, the control unit 310 transmitscontrol signals to the DC/AC inverter 306.

The frequency of the square-wave alternating current can be varied in adesired manner by altering settings values in the program.

When a lighting switch of the lamp 100, which is not shown in FIG. 3, isswitched on, the lighting device 300 applies a high voltage pulse to thelamp 100. Subsequently, when dielectric breakdown occurs between theelectrodes of the lamp 100 and an arc discharge current begins to flowbetween them, the current detector 312 transmits a detection signal tothe control unit 310, and a lighting discrimination circuit in thecontrol unit 310 judges that lighting has started.

After judging that lighting has started, the control unit 310 performscurrent control, keeping the current at a predetermined level until thevoltage increases and the lamp 100 reaches its power rating. Then, whenthe voltage has reached a predetermined value, the control unit 310switches over to rated power control. Namely, the control unit 310compares a product of a current value detected by the current detector312 and a voltage value detected by the voltage detection unit 314 witha power criterion value stored in the memory of the control unit 310,and controls the output current from the DC/DC converter 304 to give thepredetermined power.

Note that the control unit 310 is connected to a switch forcommunicating with a control unit 402 (see FIG. 10) that is providedexternally to the lighting device 300.

4. Sequence of Events Leading to the Invention of the PresentApplication

The invention of the present application was arrived at by anexamination of results from lighting tests using a conventionaltechnique of fixing the frequency of the square-wave alternating currentflowing during lighting, and by an examination of a conventionaltechnique for controlling the electrode tip protuberances, and these aredescribed below in the stated order.

(1) Examination of Lighting Test Results when the Frequency was Fixed

The inventors set different values for the fixed frequency of thesquare-wave alternating current flowing during lighting in a numberlamps 100, and performed lighting tests to investigate the temporalvariation of a luminance sustain ratio and lamp voltage of each lamp100.

These tests were performed with lamp units 200 equipped with lamps 100with a rated power of 130 W, and the values of fixed frequency for thesquare-wave current flowing in the lamps during steady-state lightingwere set to be 85 Hz, 170 Hz, 340 Hz and 510 Hz respectively.

FIGS. 4A and 4B show graphs of these test results. In the tests, fivelamps were used at each frequency, and the resultant average values fromeach set of five lamps were plotted in the graphs.

In the graph of FIG. 4A, the luminance sustain ratio is expressed aspercentage of the luminance in an initial lighting period during whichthe luminance sustain ratio is said to be 100%. This measure ofluminance is based on the ANSI lm measurement (ANSI lumen measurementthe intensity of lighting projected onto screen by an optical system ismeasured at 9 predetermined points, and the ANSI lumen value appraisedby calculating a luminous flux from the average intensity). Further, thegraphs of FIG. 4 are semi-logarithmic with the total lighting time ofthe horizontal axis being expressed logarithmically. This is done tomake it easier to see the dynamic variation during the initial stages oflighting.

As shown in the graph of FIG. 4, though the luminance of the lamp forwhich the fixed frequency was set to 85 Hz increased substantiallyimmediately after lighting was begun, it peaked after approximately 20hours of lighting and dropped sharply thereafter. This drop in luminanceis caused by the electrode protuberances growing excessively and theinternal wall of the are tube 101 blackening within a short period. Notethat the lighting tests on the lamps for which the fixed frequency was85 Hz were discontinued after the deterioration due to blackening becameconspicuous.

The luminance values of the lamps for which fixed frequencies were setto 340 Hz and 510 Hz dropped immediately after the lighting began, andafter 100 hours and before 1000 hours respectively, had fallen to aluminance sustain ration of 50%, the criterion for the life.

The luminance of the lamps for which the constant frequency was 170 Hzincreased gradually immediately after lighting began, but after peakingat around 20 hours of lighting, it reduced gently, giving a life ofapproximately 3000 hours.

In the lighting tests, observation of the tips of the electrodes 102 and103 of the lamp with the longest life for which the fixed frequency was170 Hz revealed variation in the sizes of the protuberances 124 and 134of the electrodes 102 and 103.

FIGS. 5A-5C are generalized diagrams showing temporal variation of theprotuberances 124 and 134 of the electrodes 102 and 103 in a lamp 100using a conventional fixed frequency of 170 Hz.

The electrodes 102 and 103 were obtained by (i) installing tungsten wirecoils 123 and 133 at the tips of electrode axes 121 and 131, (ii)incorporating the tips of the electrode axes 121 and 121 and the coils123 and 133 into the lamp 100, after first melting and processing a partof the coils to form the semi-spherical electrode tips 122 and 132, and(iii) forming the protuberances 124 and 134 on the electrode tips 122and 132 by operating the lamp 100 by causing an alternating current of aprescribed frequency to flow for a prescribed period (see JapaneseLaid-open Patent Application No. 2001-312997 for details). Note thatparts that are “substantially spherical” may be used as the electrodetips.

As shown in FIG. 5A, before beginning the lighting test theprotuberances 124 and 134 of the electrodes 102 and 103 were of anappropriate length. However, as shown in FIG. 5B, in the first few tensof hours after the beginning of lighting, the initial stages of thelife, the protuberances 124 and 134 of the electrodes 102 and 103 grewexcessively. Subsequently, as shown in FIG. 5C, in the middle and latterstages of the life (between 2000 and 3000 hours), the protuberances 124and 134 of the electrode 102 and 103 were found to all but disappear.Note that FIG. 5C shows a general case in which the protuberances haveentirely disappeared.

The causes of the excessive growth and disappearance of theprotuberances 124 and 134 can be inferred as follows. As describedabove, a halogen material is contained inside the arc tube 101 of thelamp 100 in order to realize the halogen cycle. During lighting, whenthe tungsten that composes the electrodes 102 and 103 evaporates, itchemically combines with the halogen, and is returned by means of aconvection current inside the are tube 101 to the arc plasma, where itdissociates from the halogen and becomes plasma ions. Having becomeplasma ions, the tungsten is attracted towards a region centered aroundthe arc spot, which is the point of highest electric field concentrationand is located at the cathodic phase-side electrode tip of theelectrodes 102 and 103, where it accumulates. Then, when the cathodicphase-side electrode has reverted to its anodic phase, the temperatureof the electrode tip increases due to colliding electrons, and thetungsten accumulated there during the cathodic phase once againevaporates.

If the balance between this accumulation and evaporation is maintained,the protuberances 124 and 134 at the tips of the electrodes 102 and 103neither grow nor disappear, and they can be kept at an appropriate size.It follows that if this balance is not maintained the size of theprotuberances 124 and 134 at the tips of the electrodes 102 and 103 willvary.

It is considered that that the protuberances 124 and 134 grewexcessively in the initial stages of the life because large quantitiesof tungsten accumulated at the portions of the protuberances 124 and 134that form the arc starting points.

Since the excessive growth of the protuberances 124 and 134 leads to ashortening of the electrode gap distance De, the lamp 100 makes acorresponding move towards being a point source, the condensingefficiency of the lamp 100 in combination with the concave minor 203improves, and the luminance increases.

In the lighting test of the lamp using the fixed frequency of 170 Hz(see FIG. 4A), the luminance improved until approximately 20 hours afterthe beginning of lighting due to the shortening of the electrode gapdistance.

Note that since the lamp has power control which maintains the ratedpower, as the electrode gap distance shortens the current valueincreases and the voltage value decreases. Thus, in the lighting tests,the voltage value was particularly low approximately 20 hours after thebeginning of lighting because the electrode gap distance is short inthis period.

Subsequently, in the middle and latter stages of the life,recrystallization of the electrodes 102 and 103 begins, and because thecomposition of the electrode 102 and 103 alters, it becomes moredifficult for evaporated tungsten to return to the tips of theelectrodes 102 and 103. Hence, the protuberances 124 and 134 shorten andultimately disappear. Moreover, since tungsten continues to evaporatefrom the electrode tips 122 and 132 even after the protuberances 124 and134 have disappeared, the electrode tips 122 and 132 are graduallyeroded.

Since the disappearance of the protuberances 124 and 134 leads to alengthening of the electrode gap distance De (the arc-length increases)and the lamp 100 gradually moves away from being an ideal point lightsource, the condensing efficiency of the lamp 100 in combination thereflecting minor 203 drops, and the luminance drops accordingly.

In the lighting test of the lamp for which the fixed frequency was 170Hz (see FIG. 4A), the monotonic decline in luminance in the middle andlatter stages of the life is caused by tips of the electrodes 102 and103 eroding and the electrode gap distance lengthening.

(2) Examination of a Method to Control the Protuberances of theElectrode Tips

The problems with the prior-art method for controlling the protuberancesof the electrode tips are as follows.

Firstly, a method for intentionally promoting growth in theprotuberances at the manufacturing stage by selecting a fixed frequencyof alternating current to be supplied is disclosed in Japanese Laid-openPatent Application No. 2001-312997. In this method, stable luminanceduring lighting is achieved by shortening the electrode gap distancethrough forming suitable protuberances, making a point light source.High-pressure mercury lamps currently in use are designed to activelysupport the growth of electrodes protuberances via the halogen cycle,and through forming these protuberances, to concentrate the arcdischarge in order to give a higher lamp luminance.

Further, a method which enables the growth of protuberances to besuppressed in the initial stages of the life, which span betweenapproximately 100 hours and 500 hours, by altering the fixed frequencyof the supplied alternating current is disclosed in Japanese Laid-openPatent Application No. 2003-338394. According to this method, it ispossible to suppress excessive generation of protuberances in theinitial stages of the life, which span between approximately 100 hoursand 500 hours, enabling a stable luminance to be achieved, and thismethod has therefore been used to solve this kind of problem.

Since this method of protuberance control during the life involvesdetecting the discharge voltage during normal driving of the lamp,frequency variation is not begun without there being fluctuation in thedischarge voltage. Thus, with this method, frequency variation controlis begun only after the state of the electrode tips of the lamp hasaltered from a state of initial use.

In other words, this is a method which depends on restoration controlafter the form of the protuberances has altered, and by which frequencyvariation control is carried out after protuberances have grownexcessively in such a way that the protuberances that have grown arecaused to evaporate and the electrodes are returned to their originalstate.

One of the problems with this method is that it is difficult toeffectively control the process of slow reduction with driving time inthe size of the electrode protuberances. In the method of the prior art,control of this process is performed based on lamp operating data valueswhich alter over time, such as the lamp voltage. However, if theoperating data being detected alter slowly, it is difficult to controlthis process effectively. Moreover, since the electrical properties ofthe lamp gently change with the elapsed driving time, the optimumconditions for controlling the growth and reduction of the protuberancesalso change. Thus, the driving frequency, control period and othercontrol conditions, which have been set in the initial period for thepurposes of control, sometimes deviate from the optimum conditions.Hence, the problem arises that the protuberances cannot be effectivelycontrolled by means of the method of protuberance control during thelife.

In the invention, to tackle this problem, the frequency of thealternating current supplied to the lamp is constantly varied withoutreferring to the lamp operating data values that alter with the elapsedlighting time of the lamp, and the electrode protuberances are therebystabilized in their manufactured form, enabling the life to be extended.

It is clear from experiments that the optimum conditions forfacilitating the growth and disappearance of the protuberances of theelectrode tips vary according to such factors as the conditions of lampdesign and, when steady-state lighting takes place, the accumulatedlighting time. In view of this, an experiment in which the operablefrequency was constantly varied was attempted, and it was found thatvarying the frequency in this way had the effect of maintaining theinitial form of the electrodes.

5. Illumination Method of the Embodiment

FIG. 6 shows the square-wave alternating current generated by the lamplighting device of the embodiment. The upper graph shows the variationof the square-wave alternating current with time. The lower graphcorresponds to the upper graph and shows the variation of the frequencyof the square-wave alternating current with time.

In the embodiment, during steady-state lighting the square-wavealternating current frequency is switched in steps to each of 340 Hz,255 Hz, 170 Hz, 128 Hz, and 85 Hz respectively, the switching takingafter each square wave cycle.

When the frequency has been reduced in steps from the maximum frequencyof 340 Hz to the minimum frequency of 85 Hz, it once again increases insteps to 340 Hz, and this frequency switching is repeated.

As shown in FIG. 6, the repeated frequency switching cycle is called avariable cycle. During lighting, the frequency switching in the variablecycle is repeated periodically. Note that this variable cycle has aperiod of approximately 50.0 ms.

Illumination tests resembling those described above but making use ofthe lamp lighting method of the invention were performed.

FIGS. 7A and 7B show graphs of these test results. In the same figure, aplot for the 170 Hz fixed frequency method of the prior art is shown tofacilitate comparison.

As shown in the graph of FIG. 7A, with the variable method of theembodiment in the initial stages of the life there is no increase inluminance of the kind seen with the fixed frequency method, and theluminance is substantially constant from the beginning of lighting untilthe 100 hour mark. A constant luminance over the life is generallydemanded in lamps, and with the variable method it was possible toachieve a stability in luminance over the life which was greater thanthat of the prior art.

Further, it was seen that, with the variable method, the luminancedecreased more gently in the middle and latter stages of the life thanwith the prior art, and that an extended life of more than 6000 hourscould be realized.

It is thought that the life could be extended in this way with thelighting method of the present invention because it was possible, bymaintaining the balance between the above-described accumulation andevaporation, to maintain the form of the protrusions 124 and 134 at thetip of the electrodes 102 and 103 in the initial state of first use fora longer period than with the prior art.

FIG. 8 and FIG. 9 are tables indicating the luminance transition and thevoltage transition respectively from the above-described lighting tests.

Frequency Switching

(1) In the embodiment, the frequency is not varied directly between 340Hz and 85 Hz. There are the three frequencies of 255 Hz, 170 Hz, and 128Hz between 340 Hz and 85 Hz, and the frequency is varied in steps,taking values of 340 Hz, 255 Hz, 170 Hz, 128 Hz, and 85 Hz respectively.This method is used because if the frequency is varied abruptly, squarewaves will sometimes distort due the properties of lighting circuits. Byvarying the frequency in steps in this way it is possible to suppressdistortion of the square wave.

(2) In the embodiment, the frequency is varied between 340 Hz and 85 Hzso as to straddle the 170 Hz frequency which, among the fixedfrequencies, gave the longest life. It is thought that varying thefrequency between values straddling 170 Hz, the optimal value in termsof life, enables the balance between accumulation and evaporation to bemaintained better than with the prior art, and thereby enables the lifeof the lamp to be reliably extended. Here, “optimal in terms of life” isused to mean that in the first part of the life a luminous flux withoutany usage problems is obtained, and in addition, that the luminous fluxmaintenance factor does not fall for an extended period.

Which pattern should be chosen for varying the frequency of thesquare-wave alternating current during steady-state lamp lighting willvary depending on the make-up of the lamp (the volume of the arc tube,the composition of the materials contained in the tube, the electrodegap distance, and the like), and a suitable pattern can be determined byexperiment.

(3) The Lower Limit of Frequency Variation

When the lamp 100 is being driven for steady-state lighting, a minimumfrequency of several Hz can be used. However, if the minimum frequencyof the flashing of the lighting device and projector display screen is50 Hz or 60 Hz, the frequencies at which alternating current is suppliedcommercially, the display screen will appear to flash. In order to avoidsuch an undesirable state, the commercial frequencies must be avoidedand, in order to do this, a minimum frequency of 60 Hz or more isrequired. Experiments show that, when the frequency is set 10 Hz abovethe commercial frequencies, such flashing reduces to the point where itis no longer noticeable. Hence, when a margin with respect to thecurrent maximum commercial-use frequency has been taken into account,frequencies of 70 Hz or more can be selected as safe frequencies. Notethat it has been confirmed that the protuberances at the electrode tipwill grow at these frequencies.

(4) The Upper Limit of Frequency Variation

It has been confirmed that, when the 130 W lamp 100 is driven at 130 W,steady-state driving is possible at each of the fixed frequencies of 300Hz, 400 Hz, 500 Hz, and 550 Hz. Experiments were also carried out withregard to other electrical conditions, and it was found that anyfrequency between 300 Hz and 500 Hz inclusive could be used as themaximum fixed frequency. Moreover, when variation among individual lampswas taken into consideration, the maximum frequency at whichsteady-state driving was possible also fell in the range of 300 Hz to500 Hz inclusive. The electrode protuberances of the lamp were alsoassessed at this time. With this range of fixed frequencies, rather thangrowing, the electrode protuberances of the lamp are seen to tend toreduce in size. This is confirmed by the lamp discharge voltage data. Inother words, at these frequencies, either the growth of the generatedprotuberances at the electrode tips is minimal, or the electrode gapdistance increases.

(5) It is desirable that the maximum frequency to which the frequency isto be varied is at least 3 times the minimum frequency. This is becauseit is thought that if a difference of approximately this magnitude isestablished between the minimum and maximum frequencies, it is easier tomaintain the electrode gap distance appropriately.

(6) In the embodiment, the frequency is switched among the threefrequencies of 340 Hz, 170 Hz, and 85 Hz, but it may instead be switchedbetween only two frequencies. When this is the case it is desirable, inorder to the extend the period that the electrode gap distance isappropriately maintained beyond that of the prior art, to set the higherof the two frequencies to a frequency at which the electrode tips growin the initial stages of the life, and to set the lower frequency to afrequency at which the electrode tips reduce in size in the initialstages of the life.

6. Image Display Device

The lamp unit 200 which is of the invention and which includes the lamplighting device 300 can be applied in a projector-type image displaydevice.

FIG. 10 is a schematic diagram showing the structure of a liquid crystalprojector 400 using the above-described lamp unit 200, which is oneexample of such a projector-type image display device.

As shown in FIG. 10, the transmission-type liquid crystal projector 400is composed of a power source unit 401, a control unit 402, a condenserlens 403, a transmission-type color liquid crystal display panel 404, alens unit 405 which contains a driving motor, and a fan device 406 forcooling purposes.

The power source unit 401 transforms commercial-use AC input (100V) to apredetermined DC voltage, and supplies the DC voltage to the controlunit 402.

The control unit 402 drives the color liquid crystal display panel 404,causing it to display color images based on image signals inputted fromthe exterior. Further, the control unit 402 controls the driving motorinside the lens unit 405, causing the lens unit 405 to execute focusingoperations and zoom operations.

Light irradiated from the lamp unit 200 is condensed by the condenserlens 403, and transmitted through the color liquid crystal display panel404, which is disposed in the optical path, and the image formed on theliquid crystal display panel is thereby projected through the lens unit405 and onto a screen not shown in FIG. 10.

Using a lamp unit which has a longer life than those of the prior artand which is composed of the high-pressure mercury lamp and lightingdevice of the present invention enables a liquid crystal projector ofgreat commercial value can be provided.

Note that the lamp unit 200 which includes the lamp lighting device 300of the invention can be applied in other projector-type image displaydevices, such as DLP (registered trademark) style projectors that useDMPs (digital micro-mirror devices), liquid crystal projectors that useother reflection-type liquid crystal components, and the like.

7. Modifications

Though the embodiment has been described above, the invention is not ofcourse limited to the above-described embodiment and suitablemodifications are possible provided they do not depart from the scope ofthe invention.

(1) Various modifications of the pattern for switching the frequency ofthe square-wave alternating current are conceivable. FIG. 11 throughFIG. 15 show examples of such modifications. These figures all show thepatterns in the same manner as the upper graph of FIG. 6.

The switching pattern of FIG. 11 resembles those of the First Embodimentin that when the frequency has been reduced in steps from a maximumfrequency to a minimum frequency it is once again increased in steps tothe maximum frequency, but differs in that the frequency is switchedevery square-wave half-period. For example, in one variable cycle, thefrequency will be switched every square-wave half-period, taking in turnvalues of 340 Hz, 255 Hz, 170 Hz, 128 Hz, 85 Hz, 128 Hz, 170 Hz and 255Hz.

In the switching pattern of FIG. 12, one variable cycle is a period overwhich the frequency is switched from the maximum frequency to theminimum frequency in steps, a switching step taking place every fullsquare-wave period. For example, in one variable cycle, the frequencywill be switched every full square-wave period, taking in turn values of340 Hz, 255 Hz, 170 Hz, 128 Hz, and 85 Hz.

In the switching pattern of FIG. 13 the frequency is switched from themaximum frequency to the minimum frequency, a switching step takingplace every square-wave half-period. For example, in one variable cyclethe frequency will be switched every square-wave half-period, taking inturn values of 340 Hz, 255 Hz, 170 Hz, 128 Hz, 85 Hz, 128 Hz, 170 Hz and255 Hz.

In the switching pattern of FIG. 14, one variable cycle is a period overwhich the frequency is switched at regular intervals and in turn to thethree frequencies of the frequency fixed periods a, b, and c. Forexample, the fixed frequency of fixed period a may be 340 Hz, the fixedfrequency of fixed period b may be 170 Hz, and the fixed frequency offixed period c may be 85 Hz.

In the switching pattern of FIG. 15, one variable cycle is a period overwhich fixed periods for which the frequency is fixed alternate withvariable periods for which the frequency is varied. For example, onevariable cycle may be composed of two periods, the first being a fixedperiod with a fixed frequency of 340 Hz, and the second being a variableperiod during which frequency switching similar to that of the variablecycle shown in FIG. 12 is carried out.

(2) In the embodiment, the frequency was varied intermittently, but itis acceptable to vary the frequency continuously.

An lighting device for realizing continuous variation of the frequencyis described below. FIG. 16 is a block diagram showing the structure ofa lighting device 301 of a modified example. Since the lighting device301 shown in FIG. 16 has a structure that is fundamentally the same asthe lighting device 300 of the embodiment (see FIG. 3), the descriptionbelow is centered on those aspects that differ.

The oscillator 318 generates a modulation signal and the oscillator 320generates a modulating signal.

The frequency determining circuit 316 modulates the modulation signalusing the modulating signal and generates a modulated square wave.

The control unit 310 makes reference to the square wave, and transmitsthe predetermined control signal to the DC/AC inverter 306.

With this kind of lighting device 301, the frequency of the square wavealternating current supplying the lamp can be varied constantly using asimple circuit structure, and it is possible to finely control thisfrequency variation.

Note that instead of the frequency determining circuit 316 and theoscillators 318 and 320, one of the known frequency modulation circuitsmay be used.

(3) Various other modified examples are conceivable for the frequencyvariation. These modified examples are shown in FIG. 17 through FIG. 20.The graphs of FIG. 17 through FIG. 20 all have time (t) plotted on thehorizontal axis and frequency (f) plotted on the vertical axis, andindicate the variation of time with frequency.

(A) Regular Variation and Irregular Variation

The frequency may be varied regularly, which is to say according to somepredetermined rule, or be varied irregularly. The graph of FIG. 17 showsregular variation and irregular variation in a conceptual way. As shownin FIG. 17, whereas the frequency variation in period A₁ is regular, thefrequency variation in period A₂ is irregular. Note that, while thefrequency variation in the period A₂ is irregular, a maximum frequency(f max) and a minimum frequency (f min) are defined, and the frequencyis varied between these two.

(B) Intermittent Variation and Continuous Variation

The frequency may be varied intermittently, or varied continuously,which is to say constantly. The graphs of FIG. 18A and FIG. 18B showintermittent variation and continuous variation in a conceptual manner.

In the graph of FIG. 18A, a predetermined period of frequency variationis followed by a period of fixed frequency, and subsequently by furtherpredetermined periods of frequency variation interspersed with furtherfixed frequency periods.

In the graph of FIG. 18B, on the other hand, the frequency is variedcontinuously.

(C) Combination of Intermittent and Continuous Variation

Periods of the intermittent variation and continuous variation describedin (B) above may be combined. For example, periods of intermittentvariation and periods of continuous variation may be alternated, asshown in FIG. 19.

(D) Grouping of Periods of Variation and Periods of Fixed Frequency

Though not specifically shown in the drawings, a period including one ormore periods of continuous frequency variation and one or more periodsof fixed frequency may form one cycle, and this cycle may be repeated.For example, a period of continuous variation and a subsequent period offixed frequency may be grouped and treated as a single cycle, and thiscycle repeated.

(E) Irregular Repetition

As shown in the graph of FIG. 20, a period of continuous variation and aperiod of fixed frequency may be established, and these two periodsrepeated irregularly. This is to say that instead of an order beingpredetermined, which of the two periods is to follow a given period maybe left unspecified.

(F) Combination with Frequency Variation that is Dependent on OperatingData (Such as Discharge Voltage Data).

In the description of (A) through (E) above, the combination of periodsof frequency variation not dependent on the operating data and periodsof fixed frequency has been described. However, instead of the periodsof fixed frequency, it is acceptable to use periods during which thefrequency is varied according to the operating data.

(4) It is possible, by applying a combination of the lighting method ofthe invention and a lighting method by which the lighting power ischanged during steady-state lighting, to further suppress the reductionin transparency of the lamp are tube due to clouding, transformation andthe like, enabling a longer life to be achieved.

This is because, by switching to a lower power (within a range of 60% to95% of the rated power, for instance) the heat produced by the arcduring lighting can be reduced, and clouding, transformations and thelike in the are tube can be suppressed.

8. Additional Information

Though in the embodiment the example of a high-pressure mercury lampcontaining mercury as a light emitting material was described, theinvention can also be applied in other high-pressure discharge lamps,such as metal halide lamps and the like.

INDUSTRIAL APPLICABILITY

The high-pressure discharge lamp lighting device of the invention givesa longer life than the prior art and can therefore be applied in liquidcrystal display devices and the like.

1.-25. (canceled)
 26. A high-pressure discharge lamp lighting devicecomprising: a lighting circuit supplying an alternating current to ahigh-pressure discharge lamp to cause lighting, the high-pressuredischarge lamp having an arc tube in which a halogen material isenclosed and a pair of electrodes is disposed, each electrode having aprotuberance at a tip thereof, the protuberance formed through a halogencycle; and a frequency controlling unit switching, during a continuousoperation of the high-pressure discharge lamp, a frequency of thealternating current intermittently among two or more different valueswithout dependency on operating data that varies with an elapsedlighting time of the high-pressure discharge lamp, wherein a maximumfrequency of the alternating current is in a range of 300 Hz to 500 Hzinclusive, and is at least three times a minimum frequency of thealternating current.
 27. The high-pressure discharge lamp lightingdevice of claim 26, wherein the lighting circuit repeats a cyclecomposed of (i) a first period for supplying the alternating currentwith a first fixed frequency and (ii) a second period for supplying thealternating current with a second fixed frequency that is different fromthe first fixed frequency.
 28. The high-pressure discharge lamp lightingdevice of claim 27, wherein at least one of the first period and thesecond period is long enough to include a plurality of cycles of thecorresponding fixed frequency.
 29. A high-pressure discharge lampapparatus, comprising: a high-pressure discharge lamp having an arc tubein which a halogen material is enclosed and a pair of electrodes isdisposed, each electrode having a protuberance at a tip thereof; and thehigh-pressure lamp lighting device of claim
 26. 30. A projector-typeimage display apparatus comprising the high-pressure discharge lampapparatus of claim
 29. 31. A high-pressure discharge lamp lightingmethod comprising: a lighting step of supplying an alternating currentto a high-pressure discharge lamp to cause lighting, the high-pressuredischarge lamp having an arc tube in which a halogen material isenclosed and a pair of electrodes is disposed, each electrode having aprotuberance at a tip thereof, the protuberance formed through a halogencycle; and a frequency controlling step of switching, during acontinuous operation of the high-pressure discharge lamp, a frequency ofthe alternating current intermittently among two or more differentvalues without dependency on operating data that varies with an elapsedlighting time of the high-pressure discharge lamp, wherein a maximumfrequency of the alternating current is in a range of 300 Hz to 500 Hzinclusive, and is at least three time s a minimum frequency of thealternating current.